Method of detecting and controlling perceptual flicker

ABSTRACT

A method and apparatus are provided for processing light from a light source. The method includes the steps of measuring a predetermined set of characteristics of the light source and detecting flicker when the predetermined set of characteristics exceed a corresponding flicker fusion threshold value.

This application is a continuation of U.S. patent application Ser. No.12/593,816 filed on Sep. 29, 2009 (allowed), which is a national phaseapplication of PCT/US07/22211 filed on Oct. 18, 2007 (now abandoned),which is a continuation-in-part of Provisional Pat. Application No.60/949,143 filed on Jul. 11, 2007 and U.S. Provisional Pat. ApplicationNo. 60/921,959 filed on Apr. 5, 2007.

FIELD OF THE INVENTION

The field of the invention relates to visual stimulation and moreparticularly to the effects of visual stimulation on a person's brain.

BACKGROUND OF THE INVENTION

Many devices that produce artificial light do so on a discontinuousbasis. For example, incandescent or fluorescent lights produce lightfrom an alternating current power source that operates at 50-60 cyclesper second (Hz). In most cases, the artificial light produced by suchdevices appears to most people to be uninterrupted, continuous light.

In some cases, the devices that produce such light may begin tomalfunction and produce a noticeable flicker, such as for example, afluorescent light. A malfunctioning fluorescent light that flickers,however, is only a minor irritation to most people.

Similarly, television sets, video games and movies operate by dividingimages into a series of image frames that are presented to a viewer at apredetermined frame rate. In the case of television, the frame rate is30 frames per second.

In order to reduce any flicker and to produce the effect of a morecontinuous image, television sets use a concept called interleavescanning. Under interleave scanning, a television set divides the screeninto odd and even horizontal lines. To present an image, a controller ofthe television presents a first image on the odd lines followed by asecond image on the even lines followed by a third image again on theodd lines and so on.

In general, most people are not able to process information fast enoughto perceive the changing images. In most cases, the inability of thehuman visual system to process high speed optical signals operateseffectively, to make such changing optical signals appear to be acontinuous image.

However, some visual images are not always perceived as continuous. Forexample, equipment malfunction, laser battles between players in videogames or even explosions in movies can cause severe neural disruptionsin some people, leading to headaches and, in some cases, epilepticseizures. Because of the dangerous health effects, a need exists for amethod of avoiding the effects produced by such images or series ofimages.

SUMMARY

A method and apparatus are provided for processing light from a lightsource. The method includes the steps of measuring a predetermined setof characteristics of the light source and detecting flicker when thepredetermined set of characteristics exceed a corresponding flickerfusion threshold value.

In another aspect, the predetermined set of characteristics furtherincludes a frequency of light output from the light source and a dutycycle.

In another aspect, the corresponding flicker fusion value furthercomprise a lower frequency threshold of 15 Hertz with a 90% duty cycleand an upper frequency threshold of 120 Hertz with 10% duty cycle.

In another aspect, the method includes adjusting a frequency or dutycycle of the light from the light source to cause flicker fusion.

In another aspect, the method includes blocking the light from the lightsource when flicker is detected.

In another aspect, the method includes defining the light source as avideo signal.

In another aspect, the method includes providing a warning when theflicker exceeds the flicker fusion threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a set of timing charts that depict flicker fusionthreshold values under an illustrated embodiment of the invention;

FIG. 2 depicts a flicker detector that uses the threshold values of FIG.1; and

FIG. 3 depicts goggles or spectacles that use the flicker detector ofFIG. 2.

DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT

Every day billions of people worldwide operate under flickeringartificial lighting conditions, or observe flickering computer/TVdisplays and/or cinematic pictures. Given the number of people who relyon artificial lighting conditions, advances leading to even modest gainsof function in people who are extremely sensitive to flicker has asignificant impact. Although flickering optical stimuli above somethreshold may be visible to high speed optical detectors for only afraction of any time period, they appear as continuous and stable tohumans because we perceptually integrate successive flashes in a processcalled “flicker fusion” (FF). However, physiological evidence in humansand monkeys show that flicker rates above the perceptual criticalflicker frequency (CFF) threshold can nevertheless generate cortical andsubcortical visual responses. Thus the temporal integration underlyingFF does not occur at the level of the retina, but takes place later inthe visual hierarchy.

It has been found that for two brief-duration visual targets presentedin close succession, the after-discharge from the first target mayinterfere with or inhibit the onset-response from the second target(i.e., mutual suppression). In this case, the target may be a flash oflight or some other visual stimuli. In preliminary experiments, theeffects of inhibition at the termination of the first stimulus has beenstudied by presenting the stimulus twice, with varied intervals betweenthe first and second presentations. It has been found that with shortinter-stimulus intervals, both the after-discharge of the firststimulus, and the onset-response of the second stimulus, were inhibited.

The research establishes that when FF occurs perceptually, it is due tothe lack of robust firing of various neural elements in response to thesubsequent flickering stimulus (which, in a sense, is caused by thestimulus forwardly and backwardly masking itself). The duration of theinhibitory effect on perceiving subsequent flashes after the first flashcoincided in time and had the same duration as the time-out period ofthe first flash.

FIG. 1 shows the effect of the inhibitory effect on subsequent flashes.More specifically, FIG. 1 shows a series of electrical activityrecordings from upper neural layers of area V1 in a rhesus monkey inresponse to light flashes with various inter-stimulus intervals (ISIs).

The top trace, E1, beginning at time T0, shows the electrical responseto the stimulus flashing once, while, the other traces, E2-8, show theelectrical responses to a double flash of varying ISIs (i.e., the firstflash beginning at time T0 and the second flash beginning at time T1).FIG. 1 shows that the onset-response to the second flash (i.e., flickerfusion) only occurs with ISIs of 30 msec or less (equivalent to 33 Hzperiodic). If the flashes are separated by more than 30 msec, theafter-discharge of the first flash and the onset-response of the secondflash begin to recover (i.e., equivalent to <33 Hz flicker). Theseintervals roughly coincide with the CFF threshold in humans for 100%contrast stimuli in the fovea (in contrast to the monkeys used in thepreliminary studies discussed above).

It has also been found that the duration or duty cycle (i.e., the ratioof ON to OFF time) of the flash also has a profound effect upon flickerfusion. In one embodiment, the flicker fusion threshold extends from alower frequency threshold boundary (e.g., 15 Hz with a 90% duty cycle)to an upper frequency threshold boundary (e.g., 120 Hz with a 10% dutycycle). In this case, the lower threshold would be equivalent a lightflash lasting approximately 60 msec with an interval between flashes of6 msec and the upper threshold would correspond to a light flash lasting0.8 msec with an interval between flashes of 80 msec.

In other embodiments, the lower frequency threshold value approaches 0Hz with a duty cycle that approaches 100%. Similarly, the upperfrequency threshold lies above 120 Hz with a duty cycle that approaches0%.

Flickering is a problem on any of a number of light emitting devices(e.g., in light bulbs, televisions, computers, etc.). As demonstratedabove, when the varying output of a light source exceeds some limit, thevisual hierarchy of a human subject's brain fuses the varying lightoutput into the perception of a continuous light in a process calledflicker fusion.

Flicker fusion can be ensured in lighting or emitting devices byreducing the duration of dark intervals between light intervals. Thecritical frequency is between the lower frequency threshold value andthe upper frequency threshold value. Flicker fusion with a 50% dutycycle and at normal photopic lighting levels typically occursautomatically above about 33 Hz. However, flicker fusion may beaccomplished at any frequency below 33 Hz by adjusting a duty cycle oflight emission towards an upper limit.

The description provided below supports the creation of a new generationof light emitting devices without perceptible flicker. In fluorescentdevices (e.g., lights), flicker fusion may be accomplished by choosingfluorescent coatings that spread out the light emitting interval over agreater portion of the on-off cycle. In electric discharge orincandescent lighting, flicker fusion may be accomplished by rectifyingthe alternating current power supply and applying direct current to thelighting device. In the case of LEDs, including those used to illuminatevideo monitors, flicker fusion may be accomplished with simple filtersconnected across the terminals of the LED. In cathode ray tubes (CRTs),or conventional televisions, the dwell time of the electron beam can beincreased at each pixel, without necessarily increasing the frame rate.Flicker fusion may also be accomplished by adding lower level lightpulses between light pulses.

In its simplest form, flicker fusion can be accomplished in incandescentbulbs by simply supplying the bulbs from either a direct current powersupply or an alternating current power supply that operates with anappropriate flicker rate/duty cycle combination. Since most incandescentbulbs operate from 60 Hz power, this would equate to ensuring that theduty cycle is somewhere above 10%. Alternatively, a resistor-capacitornetwork may be connected across the light bulb that reduces the peakvoltage, but also increases the duration of the light pulse produced bythe incandescent light.

FIG. 2 shows a flicker detector 10 shown generally in accordance with anillustrated embodiment of the invention. The flicker detector 10 mayinclude a signal detector 14, a filter processor 16, a thresholdprocessor 18 and an output 20.

The signal detector 14 may detect optical stimulus under a number ofdifferent formats. Under a first format, the flicker detector 10 may bea portable device carried by a safety inspector (e.g., an OSHAinspector) into the work place to detect harmful flicker. In this case,the signal detector 14 may include a photodiode to detect opticalsignals 36 from lights, video equipment or other industrial processingequipment emitting optical signals into the workplace.

The filter processor 16 may include one or more filters 22, 24 thatidentify optical energy lying below the flicker fusion thresholds. Oneset of filters 22, 24 may isolate the amount of optical energy withineach of a number of different frequency ranges between the lowerfrequency threshold boundary and the upper frequency threshold boundary.A duty cycle processor 26 within the filter processor 16 may determine aduty cycle by determining a ratio of ON/OFF time for the signal in eachcorresponding frequency range between the lower and upper frequencythreshold boundaries.

Alternatively, the filter processor 16 may also determine the frequencydistribution and duty cycle using a Fast Fourier Transfatin. In thiscase, the frequency and duty cycle can be determined directly from thelocation and breadth of any peaks on a calculated FFT distribution scalebetween the lower and upper frequencies of the threshold boundary.

The threshold processor 18 may receive the frequencies and duty cyclesof any detected signals received from the filter processor 16 andperform a set of comparisons with one or more threshold values 30, 32.The one or more threshold values 30, 32 may simply be some singleminimum value below which the flicker fusion threshold may not bothermost sensitive people. Alternatively, the one or more threshold values30, 32 may be different for each frequency range between the upper andlower frequency threshold boundaries.

If the detected signals exceed one or more threshold values 30, 32(thereby indicating flicker), then the flicker detector 10 provides anoutput 20. The output 20 may be an audible alarm or a meter reading.Where the output 20 is a meter reading, the meter reading may be arelative value of the optical energy that exceeds the flicker fusionthreshold.

In another embodiment, the flicker detector 10 may be incorporated intoa set of goggles or spectacles 100, as shown in FIG. 3. In this case,the flicker detector circuit 10 of FIG. 2 is incorporated into a frame102 of the goggles or spectacles 100. The signal detector 14 of FIG. 2is replaced with a photodiode 104 disposed adjacent a set of lens 106.The filter processor 16 and threshold processor 18 of FIG. 2 aredisposed within a housing 108 and the output 20 is coupled to the lens106.

In order to protect the wearer of the goggles or spectacles 100, thelenses 106 have the ability to block flickering light. In this respect,the blocking lenses 106 may be fabricated with ferromagnetic liquidcrystal display (LCD) shutters either incorporated into and provided asan integral part of the lens or provided as a coating over the lens. Asis known, a ferromagnetic LCD shutter has 3 log units of control ofextinction (opacity) and can shutter in approximately 12 microseconds.

In use, the goggles or spectacles 100 are worn by a user in a normalmanner. The lenses may also be provided as prescription lenses. Thephotodiode 104 continually monitors and detects any light signalsimpinging upon the goggles or spectacles 100. The detected signals arefirst filtered within the filter processor 16 to identify a frequencycontent and duty cycle contained within the signals. The frequencycontent and duty cycles are compared with the thresholds by thethreshold processor 18. If the identified frequency content and dutycycle of any signal exceeds the one or more threshold values, then thethreshold processor 18 may activate the blocking lens 106 to block anyflicker from impinging upon the eyes of the wearer.

The blocking lens 106 may operate under any of a number of differentmodes. For example, if the flicker processor 10 within the goggles orspectacles 100 detects flicker, then the blocking lens 106 may close forsome time period (e.g, 10 second) or until the user turns his head awayfrom the flicker source. Alternatively, a repetition processor 34 withinthe flicker detector 10 may determine a repetition rate of the flickerand activate the blocking lens 106 at a repetition rate coincident withthe flashes to allow the user at least some limited vision whileblocking the harmful flicker.

In another embodiment, the flicker detector 10 may be used to evaluatethe safety of movies, video games or other prerecorded video signals. Inthis case, the signal detector 14 may receive the video under anappropriate format (e.g., jpeg, mpeg, radio frequency, etc.) through ahard-wired or optical connection 38.

Upon receipt of the signals, the signals detector 14 may form a seriesof video frames within a memory 36. The signal detector 14 may thencompare pixels among frames to identify the rate of change of intensityvalues for the pixels among a sequence of frames. To reduce thecomputational burden, the detection processor may identify those pixelsin each frame above a threshold value and over some time period toidentify the pixels with the greatest variability and save a sequence ofthose pixels into a respective file 40, 42 along with a time indicatorof a source frame. The files 40, 42 may, in turn, be transferred to thefilter processor 16.

The filter processor 16 may apply the filters 22, 24 to each file 40, 42and among the files to identify flicker in any one location within theframes or flicker that manifests itself across multiple locations withinthe frames. It should be noted in this regard that the frame rate ortime differences of the respective frames provide the time base fordetermining the frequency of the pixel changes and duty cycle of thepixel changes. The intensity values of the pixels provides the energyvalues for comparison with the respective thresholds.

The determined frequencies, duty cycles and energy values may, in turn,be transferred to the threshold processor 18. Within the thresholdprocessor 18, the determined frequencies, duty cycles and energy valuesmay be compared with the respective thresholds of the flicker fusionthresholds to determine if the video is safe for flicker sensitivepeople to watch.

The output 20 provided by the threshold processor 18 may simply be avalue indicating a rejection or acceptance. Alternatively, the outputmay be a time log of frames that may need to be deleted to make thevideo safe for flicker sensitive people.

In another alternative, the flicker detector 10 may be incorporated intoflicker producing devices to reduce flicker at the source. For videodevices (e.g., television sets, computer monitors, etc.), the flickerdetector 10 may receive a video input for direct processing. Whereflicker is detected, the flicker detector 10 may simply forward a pixellocation and frame identifier to a controller of the video device. Inresponse, the controller may reduce the intensity level of the pixel tosome minimal threshold level or simply delete the offending frames.

In the case of lighting devices, the flicker detector 10 may operatedifferently. For example, in light dimming circuits, the flickerdetector 10 may simply block any reduction in dimming once flicker isdetected. In lighting fixtures without such controls, the flickerdetector 10 may simply deactivate the fixture.

A specific embodiment of method and apparatus for detecting flicker hasbeen described for the purpose of illustrating the manner in which theinvention is made and used. It should be understood that theimplementation of other variations and modifications of the inventionand its various aspects will be apparent to one skilled in the art, andthat the invention is not limited by the specific embodiments described.Therefore, it is contemplated to cover the present invention and any andall modifications, variations, or equivalents that fall within the truespirit and scope of the basic underlying principles disclosed andclaimed herein.

The invention claimed is:
 1. A method comprising: measuring frequencyand duty cycle of a light source operating in a frequency range between0 Hertz and above 120 Hertz; a threshold processor identifying a flickerfusion threshold condition corresponding to the measured frequency andduty cycle of the light source where the flicker fusion thresholdcondition has a respective frequency value and duty cycle value andwhere the value of one of the frequency and duty cycle of the flickerfusion threshold condition corresponds to a value of a respective one ofthe measured frequency and duty cycle; and the threshold processordetecting flicker when the other value of the measured frequency andduty cycle exceeds the other value of the frequency and duty cycle ofthe flicker fusion threshold condition.
 2. The method of processinglight as in claim 1 further comprising adjusting a frequency or dutycycle of the light from the light source to cause flicker fusion.
 3. Amethod comprising: providing a light source, the light source providesflashes of light at a predetermined frequency between 0 Hertz and 120Hertz; a first processor identifying a duty cycle associated withflicker fusion at the predetermined frequency; and a second processoradjusting a duty cycle of the flashes to cause flicker fusion.
 4. Amethod comprising: a light source, the light source providing flashes oflight at a frequency between 0 Hertz and 120 Hertz and with a dutycycle; a first processor identifying a flicker fusion thresholdcondition corresponding to one of the frequency and duty cycle of thelight source; and a second processor adjusting a value of the other ofthe frequency and duty cycle of the flashes to correspond to thefrequency fusion threshold condition thereby causing a perception of theflashes to fuse wherein the visual hierarchy of a human subject's brainfuses the varying light output into the perception of continuous light.